I think I experienced similar behavior using the translation list view, setting a value from one translation and clicking on another one sets the previous value to the new selected row.
IIRC it also affects sao.

I can confirm the bug reproducing the scenario described in msg45402.
More precisely:
- Create the two inventory lines as described
- Change the quantity in the *first* line
- Click on the second line
- Save or refresh
-> quantity in second line is set to quantity of first line.

Tested in tryton 4.8.8
Steps to reproduce:
1. Create a new inventory with two lines and save.
2. Change quantity in one line, click and focus the other line and save.
The quantity of the line changed is also modified in the line with focus.